New three phase motor winding burn out

[brizospark]

We have been involved in the installation of a dust extraction system which is powered by a fairly large 3 phase motor (18A).

The motor was pre-wire to a control box on the side of the extraction housing and the star/delta starter was also pre-wired. The only wiring that was required was the 2 sets of cables from the extraction junction box to the star delta starter. These cables were to be connected to the main contactor and the star contactor, anyway we followed the drawing to a t.

The overload was set at 22a and we have used a 32 TP type C MCB at the DB.

The motor was first operated yesterday, it ran for a few hours and then the MCB tripped. I couldn't understand why this would trip before the O/L but anyway it did.

So I had a look at the motor and it was red hot, couldn't even touch it for any period of time so we waited a bit and checked everything, our wiring was spot to drawing and everything in the starter looked ok. I took the cover off the motor when it was cool enough and checked the resistance across the set of windings and one set was Open circuit.

Any ideas what could have caused this apart from a faulty motor?

 

[Darkwood]

Your post 23 was ambiguous it can be read both ways - but that aside a motor with a failing field may be noisy depending how it failed what i was getting at was a reversed field winding, this would still run but very noisy but it would be evident from the start as it is it was commissioned and sounded ok so not the cause.

Did you not clamp test it on commission to ensure everything was as should be?...

Id get the suppliers in to fit and commission new motor don't take any second options as they have a responsibility here at a minimum to send a replacement motor...can you can you confirm the Kw rating also it may show other probabilities as to the cause.

Im just trying to establish the motor FLC is fully protected by the O/L

 

[Marvo]

So what readings did you get for line and phase current?
......

...As per Tony's question what were the running currents when you commissioned? Did you check rotation was correct?

......You still didn't give info on the run currents, how close were they to the maximum allowed 18A?

.......Did you not clamp test it on commission to ensure everything was as should be?...

Good luck with that question, it's been asked several times and still no answer so I'm assuming there were no run current tests taken.

 

[Darkwood]

In defense of the OP here .. if he's just connected the supply up to an all in one manufacture commissioned unit then although i always check myself its not always routine to double check motor running loads when they are delivered in such a set up ...but i somehow don't think this is the issues ... its a factory supplied already connected unit and just needs the OP to tag the motor up with well labelled cables the O/L is pre-set at factory and sounds correct for running current he specifies and at a guess its an 18.5Kw motor (OP to confirm). We either have a loose connection or motor fault IMHO as the cause.

 

[Marvo]

My first guess was a 10kW if the FLA is stated on the plate as 18A assuming it's a 4-pole@ 50Hz.

I'm not sure how we can rule out common or garden overload damage without a measured run current.

I'd pop the rear bearing housing off the motor and check the windings. If they're all dark brown then probably overload, if only two windings discoloured then possibly poor connection on one phase or one phase went completely open circuit. The problem I have with the loose connection theory is that any connection carrying 18A or thereabouts that was loose for any length of time would almost certainly be obviously damaged with discolouration of the termination and the wire itself. I'd also need more evidence for the faulty motor theory, if it ran fine for hours and the bearings are still free then I'd have it down as a long shot.

 

[Darkwood]

Its star/delta and O/L in delta side so will be max 0.58 of flc thus my 18.5kw guess which will have flc of approx 38amps meaning current O/L setting will be max 22amp which relates to what op says .

 

[Marvo]

Was the 18A measured or was that the value on the info plate?

That information was in the documentation supplied with the motor

I understood this 18A figure was the maximum permissible run current from the motor specs so the overload rating should be 18A X 0.58
Apologies if I have the wrong end of the stick, the English terminology sometimes causes me confusion

 

[Darkwood]

agree slight confusion but factory set OL at 22 amps must be .58 of flc as he said it was on delta config so thats where i get my 18.5kw motor ...... instructions say running current 18 amp which can lead u to believe thats on the supply so contradicts the OL setting hence i keep asking what size motor is ..... we are both right without further info ..and op disappeared.

 

[Knobhead]

Unless we get some meaningful figures we may as well give up. I don’t guess when it comes to motors.

Re an earlier post:
A reversed phase is an unusual fault. I’ve had it only twice in 40 years, each time the motor was a rewind.
A reverse phase on a Y∆, the motor wouldn’t start as the torque would be far too low to get a fan moving.

 

[Bobster]

I don't think it was even a reversed phase, as you say Tony it's a rare fault.

I think somewhere it has been wired so that when the delta contactor kicks in, only two phases are connected correctly. The motor would carry on running single phasing. From what I can see the overload was set incorrectly so it carry on running. Then burned out.

 

[Darkwood]

But that doesn't tally with the info he received it has a running current of 18 amp and a O/L setting of 22amp, given his info and the fact the Star/delta starter has the preset O/L set at 22amps in the delta format (assuming the flc of motor) then this would make the motor an 18.5kw with a full flc of about 38amps thus 0.58% O/L setting 22amps and the running load 18amps

 

[Knobhead]

To be honest RoB if the OP can’t provide the required information then we may as well wash our hands of it. I can’t see a way forward with what we have.

To the OP:
If this is a turnkey project get the machine manufacturer in and stop wasting everyone’s time.

 

[Brightspark2]

To be honest RoB if the OP can’t provide the required information then we may as well wash our hands of it. I can’t see a way forward with what we have.

To the OP:
If this is a turnkey project get the machine manufacturer in and stop wasting everyone’s time.

Ha ha ha as polite as always!...

 

[Knobhead]

Ha ha ha as polite as always!...

I also smile as I push the knife home.

Does it cost me anything to be polite?

 

[Brightspark2]

Does it cost me anything to be polite?

Just your reputation lol...

 

[pepparz]

Generally if all settings are correct an overload trip points to a mechanical problem, and a MCB trip points to an electrical problem. Does it have to be star/delta start, I would have thought a fan even up to this size it could be DOL.